The best sex scene in the history of television is when PJ first made love in "A Winter's Tale". When I saw it for the first time, my mouth literally dropped. I had never seen so much passion, romance, and love in a sex scene. I think the fact that JJ and KH dated may have added some to it.
Mitch/Gale come in second. They were doing it on the kitchen counter and on the coffee table, etc..
Finally, probably Jen/Charlie. They got it on non stop for a week or so.